The present invention relates to a color image recording method, a color image recording apparatus and a color image recording controlling method for forming recording dots of at least three colors on a recording medium, and more particularly to a technique for suppressing a tonal fringe.
A melting type color thermal transfer recording apparatus has been proposed as a color image recording apparatus for forming recording dots of at least three different colors on a recording medium. An ink sheet to which thermally melting ink of three colors, i.e., yellow (hereinafter referred to as Y), magenta (hereinafter referred to as M) and cyan (hereinafter referred to as C) or of four colors, further including black (hereinafter referred to K) is coated in this order in the longitudinal direction is used in the melting type color thermal transfer recording apparatus. Then, the ink sheet and the transfer paper that is the recording medium are overlapped with each other, and the ink is molten by applying heat from a thermal head to the ink sheet, so that the transfer is effected on the transfer paper in the order of Y, M and C or Y, M, C and K. A thermal head is provided with a plurality of heating elements arranged in a linear manner in a main scanning direction. A time width for which a current is applied to the heating elements is controlled so that an area of the recording dots transferred to the transfer paper is controlled to thereby perform the degradation expression.
An example of an array pattern of the recording dots formed on the transfer paper by the conventional melting type color thermal transfer apparatus is shown in FIG. 26. This pattern is such that the four colors of Y, M, C and K are formed each in the main scanning direction and in the sub-scanning direction at 300 dpi (dot pitch=84.7 .mu.m). Furthermore, by displacing the recording dots in each line by 1/2 (=42.3 .mu.m) of the dot pitch in the sub-scanning direction in every other line, the heat of the thermal head is diffused so that good recording dots may be formed. In this specification, the arrangement in which the recording dots in each line are displaced in every other line in the sub-scanning direction will be referred to as one-dot staggered printing.
As shown in FIG. 27, the application of the current is started for every 10 msec for both odd and even lines, and the application of the current is stopped after a lapse of time corresponding to the gradation level. Then, a time difference of 5.0 msec is provided between the timing of starting the application of the current in the odd lines and that of starting the application of the current in the even lines. This timing is determined by synchronizing timing pulses formed for every 2.5 msec. Incidentally, the timing pulses are also in synchronism with a timing for driving a stepping motor for delivering the transfer paper. The vertical dotted lines that are common with all the lines for every 10 msec and at every 300 dpi means reference lines at equal intervals from the printing starting timing.
As shown in FIG. 26, in the case where the resolution in the sub-scanning direction is at 300 dpi, it is ideal that the recording dots are arranged exactly at intervals of 84.7 .mu.m in the sub-scanning direction. However, as far as the ink sheet and the transfer paper are mechanically transferred by using the motor, so far, it is very difficult to arrange the positions of the recording dots over the entire recording region on every transfer paper one by one without any displacement of 1 .mu.m. Accordingly, it is actually inevitable that the displacement to some extent (several .mu.m to several tens of .mu.m) occurs in the sub-scanning direction from the exact position. Furthermore, in general, the displacement occurs at random to some extent (in a range of several mm to several tens of mm) in the sub-scanning direction in each color and in each printing. FIG. 28 shows an example of an array pattern of the recording dots in the case where portions where C is relatively displaced with respect to Y and M.
In the case where the recording dots are thus displaced relative to each other, even if the printing with the degradation of 50% for each of Y, M and C, the color on the transfer paper does not become a uniform intermediate grey. The reason for this is that the recording dots of a certain color are displaced relative to the recording dots of another color in the sub-scanning direction so that the nominal tone is changed, as a result of which the color fringe (which is so called tonal moire) occurs. The occurrence of the color fringe is accelerated also by the difference in transfer property (density relative to the energy) between the case the ink is directly transferred to the transfer paper and the case the ink is applied to the ink of other color.
The present invention has been made in view of the above-described circumstances, and therefore an object of the present invention is to provide a color image recording apparatus in which the recording dots of at least three kinds of color are formed and the color fringe is suppressed effectively.